Falls from ladders represent one of the leading causes of occupational injuries and fatalities. The primary factors contributing to falls from ladders are missteps and foot slips, which cause foot de-coupling from the rung leading to a fall. After a slip/misstep occurs, a person must rely on their hands as well as the foot opposite to the misstep to stop their fall and regain balance on the ladder. This response must be executed rapidly and requires coordination between the upper and lower body. Currently, there is a key gap in knowledge related to the ability of a person to recover from slips and missteps during climbing and the biomechanical response necessary to achieve this recovery using the upper or lower limbs. The overall objective of this R21 proposal is to gain quantitative knowledge on biomechanical causes and recovery response from a ladder fall as well as the role of hand strength in recovering from a ladder slip and misstep. The rationale is that successful completion of this objective will lead to improved ladder rung design and climbing practices that maximize the likelihood of a recovery and ultimately reduce ladder fall injuries and fatalities. Specific aim #1 is to characterize the sensorimotor process to a ladder misstep and slip. The first objective is to identify the sensory system that is most critical for detecting that a misstep/slip has occurred. The second objective is to identify the muscle groups that are most responsible for recovery from the perturbation. The coordinated upper- and lower- body response to slips and missteps will be characterized. To accomplish this specific aim, subjects will be exposed to both a simulated misstep and slip during ascent of a ladder by having a rung of the ladder automatically release and a rung freely spin, respectively, while available sensory input and motor response are recorded. Sensory system input will be characterized by foot forces and center of pressure for foot somatosensation, hand forces for hand somatosensation, joint angles for proprioception, and head acceleration for vestibular sensation. The biomechanical response to the misstep/slip will be characterized by recording the joint moments, muscle activity and work performed in the upper and lower body during the response. Onset and magnitude of deviations of the sensorimotor response during perturbation compared to unperturbed ladder climbing as well as the sequencing of muscle activity will be determined to identify the critical sensory system for detecting a perturbation and critical motor response fo recovery. Specific aim #2 is to determine the role of hand strength on a person's ability to recover from ladder fall. The hypothesis is that subjects' hand strength is significantly associate with the perturbation outcome (fall vs. recovery). The perturbation outcome rate will be compared across three subject groups with different levels of hand strength. The hypothesis will be supported if the low hand strength group falls more frequently than other groups. This research represents a step closer to the long-term goal to prevent fatal and disabling injuries from falls on ladders by implementing ergonomic design changes based upon biomechanical factors involved in falls.